Result so far: as FiftyTwo predicted, I've been spending time on other, higher-value distracting websites instead. My productivity is unchanged, but instead of wasting time on reddit, I'm spending time reading articles on this site or watching Starcraft or talking with friends in irc and generally doing fun things that I actually endorse. I consider this change a success so far.

EDIT MAY 1: This seems to have led to a permanent change. I estimate I spend maybe 20 minutes on reddit a week, now.